In U.S. Pat. Nos. 3,491,597, 3,633,191 and 3,748,655, The disclosures of which are incorporated herein by reference, Engelhardt and Ebel have described apparatus for monitoring the conductor temperatures of a high voltage cable utilizing an insulated pilot wire that is in thermal contact with the cable conductor. The resistance of the pilot wire, which varies with temperature, is measured by means of a bridge, the pilot wire being included within one leg thereof. Instead of adjusting the bridge resistors to obtain a null point on a galvanometer as with a classical Wheatstone bridge, the resistors have been adjusted to read null at a pilot wire temperature of zero degrees Celsius, and the degree of unbalance, indicative of the pilot wire resistance, and thus its temperature, is determined from the scale reading of a milliameter. The apparatus of the above-named patents has found its principle application in monitoring three-phase A.C. cables and in this service it has been found that where a pilot wire is incorporated in the conductor of one of the phases, currents induced in it from the other two phase conductors interfere seriously with the bridge readings, and particularly with the operation of any voltage controlled oscillator and amplifier that may be used for radio transmission of the bridge measurements. In the U.S. Pat. No. 3,748,655 it was sought to overcome this problem by means of a large capacitor across the series-connected pilot wire and return conductor. This means has, however, not proven adequate for complete elimination of interference from the induced variable currents and from the sixty hertz voltage drop in the power conductor itself, where this serves as a return lead from the far end of the pilot wire. The present invention does, however, completely eliminate this problem as well as similar problems of induced currents in long leads between a bridge and the resistive element, such as a thermocouple, being measured.